Digital electrophotographic printing products are being developed for printing high quality text and half tone images. Thus, there is a need to formulate electrophotographic toners and developers that have improved image quality. Surface treatment of toners with fine metal oxide powders, such as fumed silica or titania, results in toner and developer formulations that have improved powder flow properties and reproduce text and half tone dots more uniformly without character voids. See, for example, Schinichi Sata, et al. Study On The Surface Properties Of Polyester Color Toner, IS&T NIP13, 149-152 (1997). The improved powder fluidity of the toner or developer can, however, create unwanted print density in white image areas.
The triboelectric charging level of electrophotographic developers is known to change as prints are made. See, Nash, R. and Muller, R. N. "The effect of Toner and Carrier Composition on the Relationship between Toner Charge to Mass Ratio and Toner Concentration," IS&T NIP 13, 112-120, (1997). This instability in charging level is one of the factors that require active process control systems in electrophotographic printers in order to maintain consistent image density from print to print. Toners with a low triboelectric charge level produce prints with high reflection optical density; toners with a high triboelectric charge level produce prints with a low reflection optical density. A toner with a constant triboelectric charge level would consistently produce prints with the same reflection optical density.
What is needed in the art are toners with more stable triboelectric charge levels which decrease the incidence of dusting (defined below).